Stabilized zirconia doped with other metal ions is well-known as a good conductor of oxygen ions and it is widely being used as a solid electrolyte. It is prior art to add calcium oxide (CaO) or yttrium oxide (Y.sub.2 O.sub.3) to zirconia for improving the phase stability and the oxygen ion conductivity of zirconia to be used as a high temperature type solid electrolyte [W. Nernst, Electrochmie, 6, 41 (1900)]. Such a stabilized zirconia, however, has the drawback that, when, for instance, it is applied in a fuel cell from which much current is to be drawn, its loading characteristic as a cell declines on account of its large impedance; and to eliminate this drawback, the working temperature has to be elevated.
Meanwhile, even when such a stabilized zirconia is to be used as a solid electrolyte for an auto lambda sensor, it is an important problem to lower the impedance of the solid electrolyte itself, because this sensor is desirable to work at around 300.degree. C. Thus in the case of the conventional stabilized zirconia, its working temperature has to be elevated on account of the increased impedance.
It has also been known to use scandium oxide (SC.sub.2 O.sub.3) as an additive for lowering the impedance of zirconia [F. M. Spiridonov et al., J. Solid State Chem. 2, 430 (1970)]; but in the case of a zirconia stabilized with only scandium oxide, since the ion radius of Sc.sup.3+ is not so large as that of Zr.sup.4+, the phase stability is not enough. Accordingly, this zirconia has the drawback that it is available only for use at high temperatures and therefore it cannot be suitably used as a general sensor.